Merge when build succeeds
### What does this MR do?
Adds a button to a MR when the build/ci is running so it can be merged when/if the build is successfull.
### Are there points in the code the reviewer needs to double check?
English spelling and whether or not the grammer is correct.
### Why was this MR needed?
When you expect its all good, and don't want to revisit the current MR it can be accepted allready.
### What are the relevant issue numbers / Feature requests?
Fixes#2640 -- although `Merge if green` is replaced with `Merge when the build succeeds` the general idea is the same.
### Screenshots (if relevant)
### Further considerations
What if there are minor things needed solving, e.g. Rubocop, the current implementation will reset the approved status of the MR. It might be a consideration keep the approval for team member, or even guests. This would require an extra option in the Admin screen, though might add extra value.
#### TODO
- [x] Docs
- [x] Specs
/cc @DouweM @rspeicher
See merge request !1729
7e8dc22dd7